Implantable prosthetic bearings such as acetabular bearings, glenoid bearings, tibial bearings and the like have typically been constructed from polyethylene. Indeed, Ultra-High Molecular Weight Polyethylene (UHMWPE) is generally utilized in the construction of a prosthetic bearing due to its favorable characteristics in relation to the articulating surface of the bearing. Moreover, it has been determined that certain characteristics of UHMWPE may be enhanced by exposing UHMWPE to radiation such as gamma radiation. In particular, exposing UHMWPE to predetermined doses of radiation crosslinks the UHMWPE thereby increasing its wear resistance. As such, heretofore designed prosthetic bearings have been constructed of crosslinked UHMWPE in order to gain the aforedescribed benefits. Techniques for crosslinking, quenching, or otherwise preparing UHMWPE are described in numerous issued U.S. patents, examples of which include U.S. Pat. No. 5,728,748 (and its counterparts) issued to Sun, et al, U.S. Pat. No. 5,879,400 issued to Merrill et al, U.S. Pat. No. 6,017,975 issued to Saum, et al, U.S. Pat. No. 6,242,507 issued to Saum et al, U.S. Pat. No. 6,316,158 issued to Saum et al, U.S. Pat. No. 6,228,900 issued to Shen et al, U.S. Pat. No. 6,245,276 issued to McNulty et al, and U.S. Pat. No. 6,281,264 issued to Salovey et al. The disclosure of each of these U.S. patents is hereby incorporated by reference.
Conventional (i.e. non-crosslinked) UHMWPE also possesses a number of favorable characteristics relating to the construction of a prosthetic bearing. For example, conventional UHMWPE possesses superior ductility, toughness, and creep resistance characteristics relative to other polymers.
A prosthetic bearing is typically designed to include structures or features which perform two primary functions. Firstly, a typical prosthetic bearing design includes an articulating or bearing surface on which either a natural bone structure or a prosthetic component articulates. Secondly, a typical prosthetic bearing design also includes locking features in the form of mechanisms such as pins, tabs, tapered posts, or the like for locking or otherwise securing the bearing to either another component associated with a prosthetic assembly (e.g., a metal shell or tray) or to the bone itself.
As described above, certain polymers may have enhanced characteristics relating to one of these primary functions of the bearing (i.e., the function of providing an articulating surface), whereas other polymers may have enhanced characteristics relating to the other primary function of the bearing (i.e., the function of locking the bearing to another component or to the bone itself). What is needed, however, is a prosthetic bearing which is constructed from polymers which have enhanced characteristics relating to both primary functions of the bearing.
Another challenge associated with implantable prosthetic bearings relates to the construction of bearings which are designed to be secured directly to the bone without the use of a metal shell or tray. For example, prosthetic bearings designed completely of polyethylene may be difficult to affix to the bone with the use of bone cement since most commonly utilized bone cements do not adhere well to polyethylene. As such, a number of bearings have heretofore been designed which attempt to overcome this problem by use of a composite material. For example, a bearing disclosed in U.S. Pat. No. 5,645,594 issued to Devanathan et al includes a first layer of UHMWPE and a second layer of blended UHMWPE and poly methyl methacrylate (PMMA). PMMA is a common component in many types of bone cement. It is disclosed that the PMMA portion of the blend may be either PMMA homopolymers or PMMA copolymers. However, a blend is, by definition, non-homogeneous and is therefore often susceptible to undesirable process and product variations. What is needed therefore is a prosthetic bearing which facilitates enhanced adhesion of the bearing to bone cement when the bearing is being secured directly to bone without the use of an implanted metal shell or tray.